The present invention relates to a high-pressure process for hydrogenating aromatic amines to give mixtures of the corresponding cycloaliphatic amines and dicycloaliphatic amines in variable ratios in the presence of rhodium catalysts which may, if desired, be modified with a noble metal selected from among iridium (Ir), ruthenium (Ru), osmium (Os), palladium (Pd) or platinum (Pt) or a mixture of these metals on supports modified with oxides of chromium (Cr), molybdenum (Mo), tungsten (W), manganese (Mn) or rhenium (Re) or a mixture of these oxides.
Substituted and unsubstituted cyclohexylamines and dicyclohexylamines are used for preparing ageing inhibitors for rubbers and plastics, as corrosion inhibitors in aqueous solution and as intermediates for textile auxiliaries and crop protection agents.
It is known that cyclohexylamine can be prepared by pressure hydrogenation of aniline. For this hydrogenation, use is made mainly of noble metal catalysts, for example an Ru catalyst modified with alkali metal as described in U.S. Pat. Nos. 3,636,108, additionally using NH.sub.3 and, if desired, a solvent. A further process for the pressure hydrogenation of aniline to give cyclohexylamine is described in DE-B 1,106,319, where an Ru catalyst is likewise used. In this process, dicyclohexylamine formed together with the cyclohexylamine is added back to the starting material. However, because of the simultaneous formation of cyclohexane, the process gives only a moderate yield. According to EP-B 0,053,818, supported Pd catalysts are better than Ru catalysts; the catalysts described there contain additives which either come from a group of basic compounds of the alkali metals, alkaline earth metals and rare earth metals or a group comprising the metals Fe, Ni, Co, Mn, Zn, Cd and Ag. These catalysts allow the reduction of substituted anilines to form the corresponding cyclohexylamines, but the corresponding dicyclohexyl-amines are missing entirely. The same applies to Co catalysts which contain a basic additive (GB 969 542) and to Raney Co (JP 68/03 180).
In the processes described for the pressure hydrogenation of aniline, the dicyclohexylamine is formed only as a by-product, if at all, in addition to the cyclohexylamine. To obtain dicyclohexylamine in larger amounts, it is prepared by separate processes. Thus, for example, it can be obtained by pressure hydrogenation of diphenylamine using an Ru/Al.sub.2 O.sub.3 catalyst (DE-B 1,106,319). Furthermore, dicyclohexylamine is formed in the reaction of cyclohexanone with cyclohexylamine under a hydrogen pressure of 4 bar in the presence of Pd on carbon (FR 1,530,477).
U.S. Pat. No. 5,360,934 discloses an improved hydrogenation process in which aromatic amines are hydrogenated to their ring hydrogenated counterparts. The improvement of the process resides in the utilization of a catalyst comprising rhodium carried on a support of kappa, theta or delta alumina. U.S. Pat. No. 4,960,941 discloses an improved hydrogenation process in which aromatic amines are hydrogenated to their ring hydrogenated counterparts. The improvement of the process resides in the utilization of a catalyst comprising rhodium carried on titania support. U.S. Pat. No. 5,773,657 discloses the hydrogenation of aromatic compounds in which at least one amino group is bonded to an aromatic nucleus. The process teaches pressures above 50, preferably from 150 to 300 bar. U.S. Pat. No. 5,023,226 relates to rutheninum catalysts also containing palladium, platinum in addition to ruthenium on a support treated with chromium and manganese from the group consisting of Al.sub.2 O.sub.3 and aluminum spinel.
EP-A 0,501,265 discloses a process for preparing substituted or unsubstituted cyclohexylamine and substituted or unsubstituted dicyclohexylamine by catalytic hydrogenation of substituted or unsubstituted aniline using a catalyst containing Ru, Pd or a mixture of both metals applied to a support comprising niobic acid or tantalic acid or a mixture of the two. EP-A 503,347 discloses a further process for preparing substituted or unsubstituted cyclohexylamine and substituted or unsubstituted dicyclohexylamine by hydrogenation of a corresponding substituted aniline using a catalyst prepared by treating an .alpha.- or .gamma.-Al.sub.2 O.sub.3 as support first with at least one compound of rare earth metals and at least one compound of manganese and then with at least one Pd compound.
However, all the catalysts and processes mentioned still have disadvantages in respect of conversion, selectivity, operating life of the catalyst, the necessity of additionally using NH.sub.3, etc. A serious problem in catalyst beds for the continuous trickling-phase hydrogenation is the tendency of all previously known Ru-containing catalysts to catalyze deaminations and hydrogenolysis of the molecules to form methane as the temperatures rise. The intrinsically exothermic hydrogenation can thus, for example in the case of slight deviations from a given temperature, change over, first slightly then possibly very rapidly, into the far more strongly exothermic methanization and lead to a situation which can no longer be controlled, even as far as explosions. For this reason, very comprehensive and reliable safety precautions have to be undertaken when using Ru-containing catalysts. However, this makes the suitability of these catalysts for industrial plants questionable.
The problems which still occur today despite the progress made are shown by EP-A 0,560,127 filed in 1992: although the Ru-Pd catalysts on alkaline supports which are used here can hydrogenate aromatic amines at low pressure, they can be subjected only to low velocities of from 0.03 to 0.05 g/ml of catalyst and hour, which requires large amounts of catalyst and large reactors; NH.sub.3 has to be added in large amounts and the temperatures are held in the vicinity of 160.degree. C. Even so, hydrogenolysis, which can be recognized by the formation of benzene and cyclohexane, still always occurs despite the fact that conversion continues to be incomplete; the selectivity leaves something to be desired and the operating life of the catalysts is significantly less than, for example, in EP-A 0,324,983. Incipient deactivation of the catalyst is indicated by the slowly decreasing conversion.
It is therefore an object of the invention to provide catalysts for the industrial hydrogenation of aromatic amines to give cycloaliphatic amines, which catalysts effect complete conversion at high velocities, have a high selectivity in respect of the formation of primary and secondary cycloaliphatic amines, possess a long life and, in particular, no longer cause hydrogenolysis and methanization of the substrates.
In one complicated process, dicyclohexylamine can be obtained from the hydrogenation product of aniline over a Ni catalyst by fractional condensation. Part of the ammonia which is also formed is removed from the remaining mixture and the remainder is recirculated to the reaction (DE-C 805,518).
EP-A 0,208,933 describes Rh catalysts on supports modified with Cr-Mn salts. The catalysts were developed for dehydrogenating intermediates for o-phenylphenol formation at high temperatures.
EP-A 0,535,482 likewise describes high-temperature-resistant Rh catalysts on supports modified with Cr-Mn salts for the preparation of o-phenylphenol. The catalysts contain further noble metals in addition to the Rh. The Rh catalysts can be used in thermostatted, static catalyst beds for dehydrogenation at low pressures and at temperatures of from 300 to 400.degree. C.
It has surprisingly been found that catalysts containing Rh on specifically treated support materials are potent catalysts for achieving a process for the pressure hydrogenation of anilines, which catalysts display no tendency to catalyze the strongly exothermic hydrogenolysis to form methane, even at high temperatures and pressures, and thus ensure a high level of production safety.